X-ray systems are a commonly used medical imaging tool. X-ray systems include an X-ray tube having a cathode that acts as a negative electrode, and an anode, that acts as a positive electrode. The cathode and anode function in generating an X-ray beam that is used for imaging. Specifically, the cathode contains one or more filaments that, when subject to high voltage, release electrons via thermionic emission in the form of an electron beam. The number of filaments employed depends on the particular cathode design. The filaments are typically inset in channels of a cup structure that is part of the cathode, i.e., a cathode cup, that serves to focus the electron beam towards the anode. A portion of the electron beam impacting the anode is refracted off as an X-ray beam.
The size of the focal spot produced when the X-ray beam contacts an imaging surface is indicative of the resolution of the X-ray tube. Namely, the smaller the focal spot size produced, the better the resolution. Increasing the resolution of the X-ray tube is desirable, as it enhances the quality of the images produced.
The focal spot size is measured as two dimensions, focal spot width and focal spot length. Both the focal spot width and the focal spot length can be changed by adjustments being made to the cathode. For example, the focal spot width can be changed by altering the widths of the channels in the cathode cup surrounding the filaments and/or by adjusting the depths at which the filaments are positioned within the channels, thereby altering the shape of the electron beam leaving the cathode.
The focal spot length can be changed by placing tabs of various dimensions in the channels of the cathode cup towards the ends of the filaments, to similarly alter the shape of the electron beam leaving the cathode. These tabs are added by a technician following production of the cathode cup and assembly of the X-ray tube.
The use of tabs to alter the focal spot length is a common industry practice. However, the correct placement of the tabs in the channels is a very delicate, exact and time-consuming process. Basically, technicians adjust focal spot length using tabs by trial and error. Technicians insert tabs of a certain size at a certain position in the channels and then observe the focal spot produced. Depending on the size of the focal spot produced, the technicians can alter the size and/or positioning of the tabs accordingly. This process is performed until the desired focal spot is produced. Even with the most skilled technicians, there is always some variation from one unit to another. Thus, using the tab adjustment process, inconsistencies in the performance of the X-ray tubes are inevitable.
To avoid the time and inconsistencies inherent with the use of tabs to adjust focal spot length, designers hope to eliminate tabs as part of their cathode designs. However, the problem then arises as to how to adjust focal spot length without using tabs. Therefore, techniques are needed to adjust focal spot length without using tabs.
Additionally, with conventional cathode cup designs, a certain amount of focal spot distortion is present. This distortion results in distortions in the images produced. While a certain amount of focal spot distortion is considered, by industry standards, to be acceptable (and expected), increasing demands for higher resolution images require that acceptable level to be constantly reduced. Therefore, techniques for improving image quality are needed.
For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for improved X-ray tube design.